The hearing loss experienced by people who are hard of hearing is rarely uniform across the entire audio spectrum. For example, a person's hearing may be down by only 5 dB at 500 Hz, and down by 20 dB at 2,000 Hz. For users with this type of hearing loss, it can be helpful to provide a compensating amount of amplification at frequencies where the user is known to have a specific amount of hearing loss. Using the above example, this compensation could be a 5 dB boost at 500 Hz and a 20 dB boost at 2,000 Hz. An underlying assumption of this approach is that intelligibility, i.e., the ability for a listener to discriminate between two essentially similar sounds, is highly correlated with the ability to perceive all frequencies in the acoustic spectrum at the correct amplitude.
Although there are electronic audio devices that allow users to adjust the spectral characteristics for themselves, typically via what are commonly referred to as “tone controls” or “graphic equalizers,” a problem with this approach when applied to telecommunication systems is that users tend to adjust the characteristics to maximize the aesthetic quality of the voice rather than the intelligibility. (The inability of hard of hearing users to self-adjust audio systems optimally is a reason why audiologists, and not the individual users, make the spectral adjustments on users' hearing aids.) But perhaps the most important reason why self-adjustment of the spectral characteristics may not yield optimal speech intelligibility for hard of hearing users is that certain types of audio degradation that are common in telecommunication systems can affect these users differently from users with normal hearing, and are best mitigated through techniques that do not rely exclusively on simple spectral compensation. Examples include the distortions introduced by audio compression (e.g., GSM or G.729), packet loss, ambient noise, transducer quality, and poor signal to noise ratio. In this context, it is important to note that the optimal mitigation strategy will differ among individuals depending on the nature of the individual's hearing loss.
A user can have an assessment performed by an audiologist to obtain an acoustic compensation “prescription” that can be applied to increase the intelligibility of communications using telephones or other electronic audio devices. In practice however, it remains difficult for the user to access and apply their particular set of audiologist determined acoustic adjustments to electronic communications. This problem is even more apparent where the user desires the ability to apply their acoustic adjustments to a wide range of electronic audio devices or communication devices and/or to communication devices on an ad hoc basis.